Rollup merge of #77324 - Aaron1011:fix/const-item-mutation-ptr, r=petrochenkov

Don't fire `const_item_mutation` lint on writes through a pointer

Fixes #77321

Author: Dylan-DPC

compiler/rustc_mir/src/transform/check_const_item_mutation.rs

@@ -60,11 +60,15 @@ impl<'a, 'tcx> Visitor<'tcx> for ConstMutationChecker<'a, 'tcx> {
             // so emitting a lint would be redundant.
             if !lhs.projection.is_empty() {
                 if let Some(def_id) = self.is_const_item(lhs.local) {
-                    self.lint_const_item_usage(def_id, loc, |lint| {
-                        let mut lint = lint.build("attempting to modify a `const` item");
-                        lint.note("each usage of a `const` item creates a new temporary - the original `const` item will not be modified");
-                        lint
-                    })
+                    // Don't lint on writes through a pointer
+                    // (e.g. `unsafe { *FOO = 0; *BAR.field = 1; }`)
+                    if !matches!(lhs.projection.last(), Some(PlaceElem::Deref)) {
+                        self.lint_const_item_usage(def_id, loc, |lint| {
+                            let mut lint = lint.build("attempting to modify a `const` item");
+                            lint.note("each usage of a `const` item creates a new temporary - the original `const` item will not be modified");
+                            lint
+                        })
+                    }
                 }
             }
             // We are looking for MIR of the form:

src/test/ui/lint/lint-const-item-mutation.rs

@@ -3,13 +3,15 @@
 struct MyStruct {
     field: bool,
     inner_array: [char; 1],
+    raw_ptr: *mut u8
 }
 impl MyStruct {
     fn use_mut(&mut self) {}
 }
 
 const ARRAY: [u8; 1] = [25];
-const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'] };
+const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'], raw_ptr: 2 as *mut u8 };
+const RAW_PTR: *mut u8 = 1 as *mut u8;
 
 fn main() {
     ARRAY[0] = 5; //~ WARN attempting to modify
@@ -18,4 +20,13 @@ fn main() {
     MY_STRUCT.use_mut(); //~ WARN taking
     &mut MY_STRUCT; //~ WARN taking
     (&mut MY_STRUCT).use_mut(); //~ WARN taking
+
+    // Test that we don't warn when writing through
+    // a raw pointer
+    // This is U.B., but this test is check-pass,
+    // so this never actually executes
+    unsafe {
+        *RAW_PTR = 0;
+        *MY_STRUCT.raw_ptr = 0;
+    }
 }

src/test/ui/lint/lint-const-item-mutation.stderr

@@ -1,89 +1,89 @@
 warning: attempting to modify a `const` item
-  --> $DIR/lint-const-item-mutation.rs:15:5
+  --> $DIR/lint-const-item-mutation.rs:17:5
    |
 LL |     ARRAY[0] = 5;
    |     ^^^^^^^^^^^^
    |
    = note: `#[warn(const_item_mutation)]` on by default
    = note: each usage of a `const` item creates a new temporary - the original `const` item will not be modified
 note: `const` item defined here
-  --> $DIR/lint-const-item-mutation.rs:11:1
+  --> $DIR/lint-const-item-mutation.rs:12:1
    |
 LL | const ARRAY: [u8; 1] = [25];
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 warning: attempting to modify a `const` item
-  --> $DIR/lint-const-item-mutation.rs:16:5
+  --> $DIR/lint-const-item-mutation.rs:18:5
    |
 LL |     MY_STRUCT.field = false;
    |     ^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: each usage of a `const` item creates a new temporary - the original `const` item will not be modified
 note: `const` item defined here
-  --> $DIR/lint-const-item-mutation.rs:12:1
+  --> $DIR/lint-const-item-mutation.rs:13:1
    |
-LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'] };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'], raw_ptr: 2 as *mut u8 };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 warning: attempting to modify a `const` item
-  --> $DIR/lint-const-item-mutation.rs:17:5
+  --> $DIR/lint-const-item-mutation.rs:19:5
    |
 LL |     MY_STRUCT.inner_array[0] = 'b';
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: each usage of a `const` item creates a new temporary - the original `const` item will not be modified
 note: `const` item defined here
-  --> $DIR/lint-const-item-mutation.rs:12:1
+  --> $DIR/lint-const-item-mutation.rs:13:1
    |
-LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'] };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'], raw_ptr: 2 as *mut u8 };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 warning: taking a mutable reference to a `const` item
-  --> $DIR/lint-const-item-mutation.rs:18:5
+  --> $DIR/lint-const-item-mutation.rs:20:5
    |
 LL |     MY_STRUCT.use_mut();
    |     ^^^^^^^^^^^^^^^^^^^
    |
    = note: each usage of a `const` item creates a new temporary
    = note: the mutable reference will refer to this temporary, not the original `const` item
 note: mutable reference created due to call to this method
-  --> $DIR/lint-const-item-mutation.rs:8:5
+  --> $DIR/lint-const-item-mutation.rs:9:5
    |
 LL |     fn use_mut(&mut self) {}
    |     ^^^^^^^^^^^^^^^^^^^^^
 note: `const` item defined here
-  --> $DIR/lint-const-item-mutation.rs:12:1
+  --> $DIR/lint-const-item-mutation.rs:13:1
    |
-LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'] };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'], raw_ptr: 2 as *mut u8 };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 warning: taking a mutable reference to a `const` item
-  --> $DIR/lint-const-item-mutation.rs:19:5
+  --> $DIR/lint-const-item-mutation.rs:21:5
    |
 LL |     &mut MY_STRUCT;
    |     ^^^^^^^^^^^^^^
    |
    = note: each usage of a `const` item creates a new temporary
    = note: the mutable reference will refer to this temporary, not the original `const` item
 note: `const` item defined here
-  --> $DIR/lint-const-item-mutation.rs:12:1
+  --> $DIR/lint-const-item-mutation.rs:13:1
    |
-LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'] };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'], raw_ptr: 2 as *mut u8 };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 warning: taking a mutable reference to a `const` item
-  --> $DIR/lint-const-item-mutation.rs:20:5
+  --> $DIR/lint-const-item-mutation.rs:22:5
    |
 LL |     (&mut MY_STRUCT).use_mut();
    |     ^^^^^^^^^^^^^^^^
    |
    = note: each usage of a `const` item creates a new temporary
    = note: the mutable reference will refer to this temporary, not the original `const` item
 note: `const` item defined here
-  --> $DIR/lint-const-item-mutation.rs:12:1
+  --> $DIR/lint-const-item-mutation.rs:13:1
    |
-LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'] };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'], raw_ptr: 2 as *mut u8 };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 warning: 6 warnings emitted